1. Field of the Invention
The invention relates to the field of environmental control systems and, in particular, to a process for determining the refrigerant charge level in a sophisticated aircraft environmental control system.
2. Description of Related Art
Modern, lightweight vapor cycle systems sometimes have a shortcoming, which is that it is difficult to detect refrigerant over- and under-charges. Previous systems incorporated receivers and sight glasses, making charge detection possible although not reliable. An efficient, modern system may employ separately controlled compressor speed, evaporator expansion valve, surge control valve, flash sub-cooler expansion valve, and condenser flow control shutters. These many “moving parts” make charge detection by traditional methods (i.e. compressor inlet superheat or condenser outlet sub-cooling measurements) unsuitable. Mechanical charge level indication systems (sight glasses, liquid level indicator, float switch and receiver tank) all rely on measuring the proportion of a liquid versus saturated vapor in a container. This container is located at a position downstream of the condenser heat exchanger that will contain saturated refrigerant (liquid and vapor).
By design, sub-cooled refrigerant (no vapor content) exits the condenser heat exchanger of this vapor cycle system. A tank located at this spot would be full of liquid refrigerant, thus there would be no level to measure. In order to create a location in this system where saturated refrigerant, consisting of a mixture of vapor and liquid, would be present, the condenser heat exchanger would have to be replaced with a separate condenser and sub-cooler. A receiver could then be located between them.
The liquid level in the receiver could then be measured by mechanical means i.e. a sight gage/level switch. For a given charge level, the level in the receiver would vary, depending on ambient conditions and system load. The weight and volume penalty associated with such a design would be undesirable in most cases.
U.S. Pat. No. 5,253,482 Heat Pump Control System by E. Murway describes a scheme in which the receiver is mounted on a weight transducer, allowing the charge level to be monitored. U.S. Pat. No. 4,601,177 Refrigerant Over-Charging System Of Closed Circuit Refrigeration Air Cooling System By M. Taino, et al. describes a valve for charging a refrigeration system, which relies on sensing the liquid level in a receiver to close the valve when the desired liquid level is reached. This scheme prevents overcharging but does not detect undercharges.
Modern vapor cycle systems may incorporate a number of sensors and a microprocessor, which reads a number of system parameters, making some computation-based approaches possible. One such approach is described in U.S. Pat. No. 5,152,152. Method Of Determining Refrigerant Levels by L. R. Brickner, et al. Here, refrigerant charge level is determined by operating the system in a special mode, then comparing the time response of evaporator temperature to “model” responses, collected in controlled conditions with known charge levels.
Yet another system is disclosed in U.S. Pat. No. 5,860,286 System Monitoring Refrigeration Charge by S. Tulpule incorporates a neural network-based approach; this particular invention takes a two-step approach using four layers. The first step is a “Kohonen” type self-organizing network (although not stated as such) consisting of an input layer and a two dimensional “hidden layer” of 4×16 neurons. Individual training patterns are learned in a cluster of neurons surrounding a central neuron. The training methodology is sometimes referred to as “competitive learning” because node weight updating is based on a competition of equally spaced center neurons to see which is initially closest to the input training pattern. There is no activation function on any of these neurons. During operation an unknown input is applied to the net, which finds the three closest stored patterns, which are further processed in the “interpolation” layer, which consists of 16 neurons using a hyperbolic tangent activation. The final single output neuron also uses a hyperbolic tangent activation function.
Thus, it is a primary object of the invention to provide a process for monitoring the charge level of a vapor cycle environmental control system.
It is another primary object of the invention to provide a process for monitoring the charge level of a vapor cycle environmental control system using a back propagation neural net.
It is a further object of the invention to provide a process for monitoring the charge level of a vapor cycle environmental control system using a neural net having a minimum of input neurons.